With the advance in high integration and high performance of semiconductor devices such as VLSI (Very Large Scale Integrated Circuit) and ULSI (Ultra Large Scale Integrated Circuit), technical requirements for a gas for plasma reaction used in the production process of these semiconductor devices are becoming increasingly strict.
As a gas for plasma reaction used in the semiconductor devices, saturated fluorocarbons such as carbon tetrafluoride and perfluorocyclobutane have heretofore been widely used. However, it is said that saturated fluorocarbon gases have a long life in the air, i.e., a life of several-thousand years or more, and exert a considerable influence upon the global warming. Therefore, various novel fluorine-containing compounds have been developed as substitutes for saturated hydrocarbons.
However, in the case when compounds having a carbon-carbon double bond in the molecule such as, for example, perfluoro-1,3-butadiene or perfluorocyclopentene is used for dry etching a silicon compound layer such as a silicon oxide layer, good selectivity to a protective film such as polysilicon film and phoptoresist film has been often difficult to obtain under many etching conditions, with the result that fine patterns were difficult to form.
Perfluoroalkyne compounds having a carbon-carbon triple bond are used as raw materials for the production of fluorine-containing polymers, and pesticides and pharmaceuticals. Several processes for the production of such perfluoroalkyne compounds have heretofore been proposed.
For example, a process for synthesizing perfluoro-2-pentyne is described in J. Am. Chem. Soc., vol. 76, p611 (1954) wherein hexachlorocyclopentadiene is treated with antimony trifluorodichloride to synthesize 1,2-dichloro-3,3,4,4,5,5-hexafluorocyclopentene; and, 2,3-dichloro-1,1,1,4,4,5,5,5-octafluoro-2-pentene as produced as a by-product in this synthetic process is dechlorinated with zinc to give perfluoro-2-pentyne. This process has a problem such that an antimony pentahalide, which is troublesome to handle, must be used for obtaining the raw material (i.e., 2,3-dichloro-1,1,1,4,4,5,5,5-octafluoro-2-pentene), and the yield of perfluoro-2-pentyne is low.
Processes for synthesizing perfluoro-2-pentyne by isomerization of perfluoro-1,2-pentadiene, perfluoro-1,4-pentadiene and perfluoro-1,3-pentadiene are described in J. Chem. Soc. (C), p454 (1969); J. Org. Chem., vol. 30, p3524 (1965); and J. Am. Chem. Soc., vol. 81, p1767 (1961). These compounds used as a raw material are not readily commercially available, and the rates of isomerizing conjugated or non-conjugated carbon-carbon double bonds in these compounds into a carbon-carbon triple bond are low.
It is further described in J. Chem. Soc. (C), p454 (1969) that perfluoro-2-pentyne was purified using a gas chromatography apparatus. However, the purity of thus-obtained perfluoro-2-pentyne is 96% at the highest. The gas chromatography apparatus cannot be used for purification in an industrial scale.
The above-mentioned synthetic processes and purifying processes for perfluoroalkyne compounds are drawn to synthesis and purification of a perfluoroalkyne compound having a chainlike structure with 5 carbon atoms, namely, perfluoro-2-pentyne. Such a perfluoroalkyne compound with a relatively low molecular weight has a moderate boiling point and good handling properties, and it is expected to be widely used. Thus, it is desired to develop an industrial process for producing this compound.